1. Field of the Invention
This invention relates to a process for electrodialytically controlling the alkali metal ions in a metal plating process and more particularly to a process for controlling the alkali metal hydroxide in a tin-plating bath, the rinse water or both.
2. Description of the Prior Art
Alkali metal stannate tin plating, using potassium or sodium stannate is a commercial method of plating tin to base metal substrates. Conventionally this process has been carried out using soluble tin anodes. The use of tin anodes causes a substantial number of control problems. If the anode current density is too low, the tin dissolves in the form of stannite or stannous tin which causes rough, poorly adhering plate. If the anode current density is too high, an insoluble film forms on the surface of the anodes and the tin does not dissolve. These problems require accurate control of the anode current density which will vary as the soluble tin anodes are consumed. This in turn limits the range of current density useable in alkali metal stannate.
An alternative to using soluble anodes is to use insoluble anodes. As there is no dissolution of the anode material, the allowable current densities are much greater and more flexible. However, plating from the solution causes the generation of two moles of alkali metal hydroxide for each mole of stannate consumed. EQU K.sub.2 Sn(OH).sub.6 .fwdarw.2KOH+Sn+2H.sub.2 O+O.sub.2
This accumulation of alkali metal hydroxide must be periodically removed from the bath. One possible method is to neutralize the alkali metal hydroxide with acid. However, neutralization with acid causes other problems. The byproduct of acid neutralization with, for example, acetic acid, is an alkali metal acetate which is an extremely soluble compound and accumulates in the plating bath. When the accumulation becomes too great, the common ion effect causes a drastic decrease in the solubility of the stannate and the bath must be discarded. Another possible alternative is to use alpha oxide sols of tin as a replacement for the stannate. Sols are disclosed in U.S. Pat. Nos. 3,346,468; 3,723,273 and 3,455,794.
The use of the alpha oxide sols while operable will only work in potassium stannate solutions and will not work in sodium stannate solutions. The alpha oxide sols have a limited shelf life and are very expensive. In addition, with the use of tin sols it is necessary to maintain a high level of free alkali, that is, from five to ten ounces per gallon. This causes difficulties in a stannate bath when the substrate is an active metal as, for example, aluminum. There is a need for a process to plate tin electrolytically utilizing insoluble anodes and preventing the accumulation of alkali metal hydroxide in the plating bath.
Where the drag out from the plating bath is excessive, there is a substantial loss of the valuable potassium stannate from the plating bath. There is a need for a process to economically recover the potassium stannate and return it to the plating bath.
In a halogen acid plating bath, oxidation caused by air introduced into the bath results in a loss of tin and fluoride compounds, the formation of sodium hydroxide and an increase in the pH of the bath. Additives to maintain the pH at desirable levels increases the dissolved solids and reduces the plating efficiency. There is a need for a process to control the pH and the alkali metal ions in a tin plating bath, the rinse water or both.